1. Field of the Invention
The invention relates to an encoding method and apparatus suited for recording digital data such as a digital audio signals.
2. Description of Prior Art
In a digital tape recorder for recording/reproducing a digital audio signal with a rotating head, for example, data is recorded on a recording medium to a saturated degree. For this reason, rewriting the data can be attained without using an erasing head by recording new data onto already recorded data directly. However, when the recording/reproducing operation of data is done with different apparatuses, there is a possibility that data to be erased is left as it is due to insufficient compatibilitY between those apparatuses.
FIG. 1 shows data recording patterns formed by overwriting data through a pair of rotating heads disposed at an opposed interval of 180.degree., for example. In FIG. 1, a track Ra indicated by a solid line shows a track formed when an apparatus A records data, and a track Rb indicated by a broken line shows a track formed when an apparatus B records data. This is a case where data recording positions of the tracks Ra and Rb do not coincide with each other, as shown in FIG. 1, because of a mechanical variation in the tape running mechanism between the apparatuses. First, data is recorded by the apparatus A to from the track Ra. Next, when new data is recorded on this data by the apparatus B, an area of the track Ra where the rotating head of the apparatus B does not scan (area indicated by an oblique line at B of FIG. 2) is left without being erased. Therefore, if a magnetic tape having the recording patterns shown in FIG. 1 is reproduced by the apparatus A, there is a possibility that unnecessary data or data (whose) reproduction is not desired is read out, because the rotating head starts scanning from an area shown by the oblique line. In the case where a digital audio signal is recorded/reproduced by a rotating head, an error detection/error correction code is usually employed. An example of encoding of the error detection/error correction is shown in FIG. 3.
In FIG. 3, audio data of (mxn) words and P parity and Q parity developed from the audio data are arranged in a two-dimensional fashion. The P parity is formed of n words in a predetermined direction of data of the two-dimensional arrangement, for example, in a oblique direction. The Q parity is formed of n words in another direction of the data of the two-dimensional arrangement, for instance, in a column direction. An error detection code such as a CRC (cyclic redundancy check) code or an error correcting code such as a Reed-Solomon code is employed to form the parity. An error check code is formed as data of one block, on the basis of n-word data disposed in the column direction of the two-dimensional arrangement data which is composed of the (mxn)-word data, and the parity (P, Q) and address data are added to the data of each column. Further, a synchronization signal is added to the block data and sequentially recorded on a track of a tape-like recording medium. As a result, data of m blocks is sequentially arranged on each track. For the error check code, the error check detecting code or the error correcting code can be used in a manner similar to the P parity and the Q parity. Meanwhile, the error check code is added to check the presence of absence of an error of each block, and the block data is handled as correct data (in the absence of an error of the block data) even when the block data that should not be reproduced exists due to the fact that data is left without being erased. For this reason, if data is read out of an area in which the data is left without being erased due to insufficient compatibility between different apparatuses as mentioned above, the data cannot be detected as unnecessary data. Consequently, a serious problem results, in that an abnormal sound is generated. Also, without being limited to such insufficiency of compatibility between the apparatuses, the same problem as that mentioned above is brought about when dust which adhered at the time of recording is dropped off at the time of partial reproduction for previously recorded data.
To solve this problem, an adoption of an identification signal (hereunder called "ID code") as described in the Japanese Patent Disclosure No. 1675/1985 laid open to public inspection has been proposed by the same applicant as the present application. The ID code is a binary code of plural bits, and a predetermined value is allotted to the ID code for every occurrence of a series of recording data. By monitoring the ID code at the time of reproduction, undesirable data mistakenly contained with reproduction data can be detected and eliminated even when no error is found by the error check code.
FIG. 4 shows an example of a recording format of data recorded by a rotating head type digital tape recorder. One block-data length has 288 bits, and an eleven-bit block synchronization signal comes at the head thereof, a thirteen-bit block address then appears, and finally parity (P, Q) of 48 bits (12 bits x 4 words) for detection and/or correction of audio data comes next.
16-word audio data L1, R1, R2, . . . L8, R8 (L: audio data of left channel, R: audio data of right channel) are positioned after the parity, and an ID code ID (8 bits) and a CRC code (16 bits) serving as an error check code are added after the audio data. The CRC code performs error detection of the remaining 261-bit data except the block synchronization signal.
Also, it can be conceived, as an alternative, that both of the ID code and the CRC code have twelve bits.
The ID code, which is indicative of a series of recording, provides high identification capability when an increased bit number is used, to eliminate the possibility that there is an accidental coincidence in ID codes between objective data and data which has not been erased. From this standpoint, it is desirable that the bit number of the ID code is made larger.
On the other hand, the probability of erroneous detection of the error check code becomes lowered with an increased bit number. Therefore, it is desirable that the bit number of the error check code be made greater. However, there is a limitation to redundancy, and it tends to be difficult to fully secure the bit number of both the ID code and the error check code.